The invention resides in a method of controlling the charge air mass flow of an internal combustion engine including an exhaust gas turbocharger with adjustable turbine geometry when, during dynamic operation, the engine load increases and the incident flow cross-section of the turbine is reduced.
The power output of an internal combustion engine is proportional to the combustion air throughput and the combustion air density. With a charger, that is, with air which is compressed before it enters the internal combustion engine, the engine power output is greater than it is with an internal combustion engine having the same combustion chamber volume at the same engine speed, but with ambient air, which is not precompressed. The air can be compressed by an exhaust gas turbocharger which consists essentially of two fluid dynamic machines. They are a turbine driven by the exhaust gas flow of an internal combustion engine and a compressor, which is driven by the turbine and pre-compresses the fresh air supplied to the turbine to a degree which depends on the speed of the compressor. The turbine and the compressor are interconnected by a common shaft and consequently rotate at the same speed. The charge air pressure downstream of the compressor and the exhaust gas pressure upstream of the turbine are coupled because of the momentum equilibrium at the common compressor shaft. The exhaust gas flow is backed up upstream of the turbine because of the charge air pressure effective on the compressor. The backup pressure of the exhaust gas flow is converted into a charge pressure of the charge air flow to the internal combustion engine in accordance with a pressure conversion ratio which is determined by the respective flow cross-sections of the turbine and the compressor.
With increasing power output of the internal combustion engine, an increasing charge air volume is needed so that a correspondingly greater charge air pressure has to be generated. With an adjustable turbine geometry, for example by adjustable guide vanes of the turbine, the gas admission flow cross-section of the turbine can be adjusted and, as a result, the backup pressure energy to be transferred by the exhaust gas turbocharger to the charge air flow can be controlled. The turbine geometry can be adjusted to any position between a fully open position with a maximum gas admission flow cross-section and a closed position with a minimum gas admission flow cross-section. For every operating state of the internal combustion engine, there is a turbine geometry, that is a guide vane position, providing for a specific gas admission flow cross-section, which results in a charge air flow volume providing for minimum fuel consumption of the internal combustion engine. The gas flow admission cross-section is reduced with increasing power output so that, with a correspondingly increased compressor power output, the charge air pressure and, consequently, the charge air mass flow to the internal combustion engine are adapted to the respective state of operation.
During dynamic operation of the internal combustion engine, that is during a load change from one to another operating state while the load is increasing, also the turbine geometry is correspondingly changed to a position with smaller exhaust gas admission flow cross-section. However, with a continuous closing movement of the turbine exhaust gas admission flow control vanes, the acceleration of the exhaust gas turbocharger is not sufficient for rapidly increasing the turbine power output. An excessively slow buildup of the charge air pressure during dynamic operations leads to combustion with insufficient air and, consequently, to an increased fuel consumption of the internal combustion engine and high exhaust gas emissions.
In order to accelerate the turbocharger rapidly it is known to immediately move the turbine geometry to a closing position when the load demand is increased. The minimal admission flow cross-section of the turbine then leads to a rapid increase in the exhaust gas back up pressure so that, as a result, the turbine and, consequently, the compressor are strongly accelerated. Subsequently, the turbine flow admission vanes are opened to assume the position providing the admission flow cross-section corresponding to the demanded stationary operating state of the internal combustion engine. The longer the turbine geometry is held in a closed position, that is, the longer maximum acceleration of the compressor is maintained, the greater the charge air pressure downstream of the compressor will be. However, the backup pressure ahead of the exhaust gas turbine increases substantially more rapidly than the charge air pressure so that, without suitable counter measures, an excessively high exhaust gas backup pressure is obtained. The high backup pressure requires increased piston energy for the discharge of the exhaust gases and results in gas change losses. This again results in a deterioration of the efficiency of the internal combustion engine.
DE 40 25 901 C1 discloses a method wherein, below a predetermined exhaust gas backup pressure value, the turbine geometry for adjusting the charge air pressure is controlled in accordance with a first performance graph and, upon exceeding the predetermined exhaust gas backup pressure value, in accordance with a second performance graph. The performance graphs are selected depending on the charge air pressure. The first performance graph corresponds to the actual change of the charge air pressure during the dynamic operating phase and the second performance graph corresponds to a fictive charge air pressure above the actual charge air pressure increase. The switching over between the performance graphs is provided for by a limit pressure switch with two setting capabilities which switch is subjected to the exhaust gas pressure, which changes the switch position when the given limit pressure is reached.
In order to achieve the rapid acceleration of the exhaust gas turbocharger and achieve thereby a steep charge air pressure increase, the turbine geometry is kept, in this known process, in a closing position until the exhaust gas pressure reaches the limit value and the limit pressure switch changes its switch position. When subsequently the turbine flow guide vanes are opened, the exhaust gas pressure drops whereupon the flow guide vanes are returned to a closing state as the exhaust gas pressure falls below the pressure limit value. With this control circuit the desired stationary operating state with a corresponding turbine geometry is finally obtained, but the exhaust gas pressure is maintained in the area of the limit value since the turbine geometry is switched back and forth as the exhaust gas pressure passes the given pressure limit. With the high exhaust gas back pressure and the charge air pressure building up comparatively slowly, there is an undesirably high pressure difference between the exhaust gas manifold and the intake manifold of the internal combustion engine which adversely affects the gas change process. The pressure difference between the exhaust gas and the charge air particularly in the early phase of the acceleration of the exhaust gas turbocharger is disadvantageous. But the known method includes no means for reducing the exhaust gas backup pressure for increasing the efficiency of the internal combustion engine and consequently, the fuel consumption.
It is the object of the present invention to provide a method of controlling the charge air mass flow of an internal combustion engine with an exhaust gas turbocharger having an adjustable turbine geometry by which, during a dynamic engine operation with a load change to a higher load stationary operating state of the internal combustion engine, the engine operating efficiency is improved.